The present invention relates generally to a vacuum relief valve that prevents a rail tank car or truck tank from collapsing when the car or tank (i) contains fluid or gases that (ii) swiftly shrink when suddenly exposed to higher and thereafter colder temperatures or (iii) whenever a tank car or truck is unloaded with a suction pump. More particularly this invention relates to a vacuum relief valve that intakes atmospheric air whenever closed tanks (i) containing contracting fluids or gases within a closed container (ii) create a vacuum that is greater than a predetermined magnitude. Most particularly the present invention relates to a biasing assembly that (i) maintains a maximum flow of the atmospheric air into a vacuum relief valve and operatively attached closed container (ii) even if the closed container vacuum doe not increase in magnitude.
Rail way tank cars and trucks that transport fluids generally comprise:                (1) low pressure (general purpose) tanks; and        (2) pressure tanks.        
The present invention exclusively addresses low pressure tanks which may require vacuum relief valves. The primary purpose of the vacuum relief valve is to allow air into the tank or truck and thereby (i) prevent or delay tank failure by implosion resulting from (ii) rapidly decreasing internal pressure. The current invention is not intended for normal loading and unloading processes, and is used exclusively under non-standard conditions.
Certain liquids and gases transported in railway tank cars or tank trucks, such as but not exclusively crude oil, toluene and styrene, are especially hazardous at elevated temperature as commodities in closed containers. During transit these gases and liquids may rapidly cool, in large part due to changing temperatures within and exterior to the tank. During this cooling process the gases and liquids, otherwise known as commodity, rapidly contract within the tank or other closed container. This contraction decreases the internal pressure within the closed container or tank to a dangerous level at which the tank may implode. Furthermore, some tank cars and tanks are unloaded with a suction pump. If proper venting of the tank is not provided, the internal pressure within the closed tank will decrease to a dangerous level at which the closed tank may implode.
Prior art vacuum relief valves do not implement constant force springs as their biasing elements. FIG. 11 graphically displays the force deflection relationships for both helical springs and constant force springs as a function of load. The relationship for the helical spring behavior to an opposing vacuum is linear. As a result there is a consistent constant deflection increment that corresponds to a consistent constant increase in vacuum. In contrast, for the constant force spring the relationship between spring deflection and vacuum is non-linear and spring deflection remains constant after a specific vacuum magnitude.
As seen in FIG. 11, the lines for the helical spring and constant force spring intersect at four inches of Hg, 0.8 inch deflection; this is the set pressure for a non-specific vacuum relief valve. At this point the vacuum relief valve begins to discharge atmospheric air into the tank. When the vacuum within the tank or other closed container just exceeds four inches Hg, the constant force spring (i) deflects to a fully open position (ii) whenever valve flow area is maximized. In contrast, a helical compression spring does not achieve the full open position until the vacuum achieves six inches Hg.
As best seen in FIG. 12, existing vacuum relief valves are generally biased to a closed pre-intake position with a single helical coil spring of the behavior displayed in FIG. 11. In this biased configuration, valve openings to the atmosphere are physically closed by mechanical barriers, such as a biased sealing component. The problem arises whenever helical coil spring valves require that a tank's internal pressure continue to decrease beyond the initial valve opening pressure for maximum valve venting capacity. As seen in FIG. 11, a progressively greater tank vacuum is necessary to fully deflect the helical spring valve beyond the initial discharge pressure. For example, Elmac Technologies displays a vacuum valve without a structure analogous to constant force springs. Http://www.elmactechnologies.com/products/pvrv-387.htm.
Other valves display features also do not solve this problem. For example, Midland Manufacturing's valve was intended to prevent damage to tank cars during routine tank loading and unloading. Http://www.midlandmfg.com/products/general-purpose-car/vacuum-relief-valve. McKenzie Valve & Machining LLC discloses a vacuum relief valve with a weather cover designed to eliminate the accidental actuation of the valve. However, the prototype comprises a single coiled helical spring as the biasing component for the valve. Http://www.mckenzievalve.com/mvpages/VRV.htm.
The current invention solves this problem with a novel biasing assembly that results in (i) a constant flow rate of air into a closed container (ii) despite a decrease in opposing tank pressure from the decreasing vacuum. This biasing assembly allows air into the valve and thereafter into the container as long as the assembly's preset pressure is less than that of the tank. This biasing assembly preferably contains two constant force springs attached to a solid weight designated as a spring block. When attached to opposing lateral sides of a spring block, these constant force springs maintain a constant maximum venting capacity at just above a preset and pre-calibrated opening pressure of the valve.
With constant force springs the valve opens fully near the initial preset discharge pressure. This valve provides a greater rate of air flow because after the initial preset discharge pressure the atmospheric air maximally flows into the tank without an increase in the magnitude of the tank vacuum. The vacuum relief valve described herein also serves as an emergency device when other conventional air valves do not open during routine tank unloading operations.
In addition to the above problem, many currently existing vacuum relief valves implement a single O-ring at a lowermost sealing disc to prevent leakage between a sealing component and the valve cavity interior wall. This sealing disc is positioned immediately below cylindrical channels. Please see FIG. 12. However, the single O-ring is easily dislodged from lowermost sealing disc whenever a tank contains a highly viscous fluid. This event occurs because one segment of the O-ring may bond to the valve body interior wall surface, while another segment sticks within the circular groove within valve sealing disc. Furthermore, conventional vacuum relief valves are vulnerable to leakage because of debris that frequently lodges between the valve seal and spring housing described infra. When this debris accumulation occurs, the prior art valves leak and this leakage may result in a non-accident release of commodity.
The vacuum relief valve described herein solves these problems by implementing either a cut flat gasket or a formed flat gasket. This circular gasket prevents this dislodgement by providing a wider area for retention. Some previous vacuum relief valves exclusively implement a formed flat gasket, but the present invention is preferably a single cut flat gasket which is less expensive. In addition, a flat cut gasket is more easily obtained and less proprietary, while a flat molded gasket is less available and more proprietary. The flat cut gasket is also more readily available in emergency situations, because gasket supply companies can produce cut gaskets immediately. In contrast, a formed gasket requires days to prepare if they are not in inventory.
The single flat cut gasket/valve seal in the current invention is not dislodged because it is compressed between (i) the valve stem sealing disc lower surface and (ii) the upper surface of a seal retainer. The stepped lower surface of the valve stem sealing disc is also a new feature that further prevents the valve seal from dislodgement. This new valve seal is further compressed and prevented from dislodging by its position between the sealing disc and the circular seal retainer. Also in the disclosed invention, the valve stem is welded to the valve stem sealing disc and this weld eliminates a secondary leakage path.
In addition to the problems described supra, vacuum relief valves are inevitably exposed to the commodity within a tank or other closed container. As a result, the springs, valve stem and elastomeric seals are fouled by tank contents that prevent or impede valve operation. To solve this problem, the current invention includes a modified baffle that reduces the area through which fluid may enter the valve interior cavity. Furthermore, previous baffles required two retaining rings to secure the baffle to the valve body cavity interior wall surface. In contrast, the present invention has a machined edge along the interior surface of the valve body cavity. This machined edge retains the upper portion of the modified baffle against the valve body cavity interior surface. This machined edge also eliminates the requirement of a second retaining ring.
The reduction in splashing commodity upon the valve seal and valve body cavity conical interior surface is achieved by the baffle, the valve seal and valve stem sealing disc. This reduction is a new feature of the current invention, as is (i) the distance of the valve seal and valve stem sealing disc lower surface from the baffle uppermost surface (ii) whenever the baffle is properly positioned within the valve body interior cavity.